The present invention generally relates to an apparatus for mixing different kinds of metallic material to make a metallic alloy and, more particularly, to a ladle structure in a metallic alloy making machine, which does not require any external aggitator or stirrer heretofore necessitated to mix one kind of metallic material, accommodated in a casting ladle in molten state and therefor having a relatively high elevated temperature, with another kind of metallic material having a boiling point lower than the elevated temperature of said one kind of metallic material and introduced thereinto in molten state.
In the manufacture of, for example, nodular graphite cast iron, magnesium having a relatively low solid solubility is added to a molten cast iron. It is well known that, during the manufacture thereof, the temperature of the molten cast iron is within the range of 1,300.degree. to 1,500.degree. C. on one hand and the magnesium to be added thereto has a melting point of about 651.degree. C. and a boiling point of about 1,110.degree. C. Because of this temperature difference, addition of magnesium to the molten cast iron under the atmospheric pressure results in that the magnesium is excessively excited to explosion. This is very hazardous to attendant workers.
Moreover, during the addition, magnesium particles to be added tend to be spattered over the ladle in contact with the molten cast iron and an insufficient amount of magnesium is consequently added to the molten cast iron. At the same time, since this addition causes the magnesium particles to transform from the solid state into the molten state absorbing heat energies evolved by the molten cast iron, the temperature of the molten cast iron tends to be lowered. In addition, in order for graphite to be spherodized, the magnesium particles must be uniformly dispersed in the molten cast iron during the manufacture.
In order to obviate the foregoing drawbacks, there has recently been contemplated a method of adding a metallic material of low boiling point to a molten metal of a temperature higher than the boiling point of the metallic material being mixed therewith. According to this method, a space above the surface level of the molten metal within a ladle is first confined by placing a lid tightly onto the top opening of the ladle and the metallic material to be added is subsequently introduced into the molten metal within the ladle. This introduction is carried out while the pressure within the space above the surface level of the molten metal is adjusted so as to attain a value 15 to 30% lower than, that is, 70 to 85% of, the vapor pressure evolved by the fusion of the metallic material being introduced. By so doing this way, a major portion of particles of the metallic material being introduced into the molten metal within the ladle can effectively be added to the molten metal without substantially being scattered out of the ladle and, simultaneously therewith, the rest of the metallic material particles are boiled in the molten metal to effect an stirring action necessary to uniformly disperse the metallic material particles into the molten metal.
More particularly, it has been found that, when magnesium is added to a molten cast iron, the vapor pressure evolved by the magnesium being added varies depending upon the temperature of the molten cast iron as tabulated below:
______________________________________ Temp. of Molten Vapor Pressure of Cast Iron (.degree. C) Magnesium (Kg/cm.sup.2) ______________________________________ 1,300 3.5 1,350 5.1 1,400 7.2 1,450 10.1 1,500 14.3 ______________________________________
If the pressure exerted within the ladle is higher than the vapor pressure at the time of addition of magnesium to the molten cast iron of a particular temperature, magnesium particles will be fused into the molten cast iron without substantially being boiled by the effect of the elevated temperature of the molten cast iron. On the other hand, if the pressure exerted within the ladle is lowered, the amount of magnesium particles to be boiled in the molten cast iron increases in proportion to reduction of the pressure exerted within the ladle.
Accordingly, it can be concluded that, if the magnesium is introduced into the molten cast iron while the pressure exerted within the ladle is properly adjused to a value adequately lower than the vapor pressure, a major portion of the magnesium particles can be fused into the molten cast iron while the rest thereof are boiled. Specifically, if the pressure exerted within the ladle is adjusted so as to attain a value lower than the vapor pressure by 15 to 30% of the latter at the time of addition of the magnesium particles to the molten cast iron within the ladle, a major amount of the magnesium particles are, as they are readily fused into the molten cast iron, mixed into the molten cast iron to effectively spherodize a graphite component of the cast iron while the rest thereof are boiled to effect a stirring action to uniformly disperse the whole magnesium particles into the molten cast iron.
In the practice of the foregoing method, however, the ladle used to accommodate a predetermined amount of molten cast iron must be of such a design that a substantially hermetically sealed space can be formed within the ladle and above the surface level of the molten metal within said ladle to allow said space to be pressurized to 70 to 85% of the vapor pressure of the magnesium being added and, at the same time, the ladle can stand against such a high pressure. Where a ladle of a conventional pressure-resisting construction is used in the practice of the foregoing method, the conventional ladle will be subjected to an abnormal pressure exceeding the designed pressure against which the conventional ladle can stand, particularly immediately after the magnesium has been added. This is because, although the temperature within a refractory container, which is used to accommodate therein a required amount of magnesium to be added and through which the magnesium is, while said container is introduced into the molten cast iron, released into the molten cast iron, is lowered under the influence of latent heat, evolved upon vaporization of the added magnesium, as the time passes and the reaction within the container is therefore brought to an equilibrium, the magnesium added is rapidly vaporized immediately after it has been introduced into the molten cast iron through the refractory container.
In view of the foregoing, the conventional ladle cannot be relied on in the practice of the foregoing method due to an insufficient physical strength with respect to the elevated temperature and rapid variation in pressure.